Tektites
Tektites (from Greek τηκτός tēktós, "molten") are -sized bodies composed of black, green, brown, or gray natural formed from terrestrial debris ejected during meteorite impacts. The term was coined by Austrian geologist (1867–1941), son of . They generally range in size from millimeters to centimeters. Millimeter-scale tektites are known as microtektites. Tektites are characterized by: # a fairly homogeneous composition # an extremely low content of water and other # an abundance of # a general lack of microscopic crystals known as microlites and chemical relation to the local or local sediments # their distribution within geographically extensive Occurrence Since 1963, the majority of known tektites have been known to occur only within four geographically extensive strewn fields: the Australasian, Central European, Ivory Coast, and North American. As summarized by Koeberl, the tektites within each strewn field are related to each other with respect to the criteria of petrological, physical, and chemical properties, as well as their age. In addition, three of the four strewn fields have been clearly linked with impact craters using those same criteria. Recognized types of tektites, grouped according to their known strewn fields, their associated craters, and ages are: * (no associated crater identified, age: 0.77–0.78 million years): ** s ( , dark, mostly black); ** s ( , dark, mostly black); ** s ( , black). * Ivory Coast strewnfield ( impact crater (10 km), Ghana, age: 1 million years): ** s (Ivory Coast, black). * Central European strewnfield ( impact crater (24 km), , age: 15 million years): ** s (Czech Republic, green). * North American strewnfield ( (40 km), United States – age: 34 million years): ** s (Texas – black to dark brown, some with metallic finish); ** s (Georgia – green). Comparing the number of known impact craters versus the number of known strewn fields, Artemieva considered essential factors such as the crater must exceed a certain diameter to produce distal ejecta, and that the event must be relatively recent. Limiting to diameters 10 km or more and younger than 50 , the study yielded a list of 13 candidate craters, of which the youngest eight are given below, Preliminary papers in the late 1970s suggested either or as the source of the . Povenmire and others have proposed the existence of an additional tektite strewn field, the Central American strewn field. Evidence for this reported tektite strewn field consists of tektites recovered from western Belize in the area of the villages of Bullet Tree Falls, Santa Familia, and Billy White. This area lies about 55 km east-southeast of Tikal, where 13 tektites, two of which were dated as being 820,000 years old, of unknown origin were found. A limited amount of evidence is interpreted as indicating that the proposed Central American strewn field likely covers Belize, Honduras, Guatemala, Nicaragua, and possibly parts of southern Mexico. The hypothesized Pantasma Impact Crater in northern Nicaragua might be the source of these tektites. Australites of molten glass in the atmosphere during .}} Australites are found in . They are mostly dark or black, and have shapes including discs and bowls that are not seen in other tektites. used the shape of "flanged button" australites in designing re-entry modules for the in the 1960s. Most scientists believe that australites formed during a large asteroid or comet impact on the Earth. The impact ejected myriads of small rocks right out of the atmosphere. The australites acquired their streamlined, aerodynamic forms when they re-entered the Earth's atmosphere while molten and travelling at high velocities. Most australites are found in Southern Australia, below 25 degrees latitude. Based on similar ages and compositions, they represent the southern edge of the vast that stretches from southern China to Australia. The Australasian strewnfield has an age of 0.8 million years and is the result of an impact in Indochina. Category:Earth